The present disclosure is directed to highly corrosion and wear-resistant metal substrates having an oxidized surface layer having chemically bonded thereto a nanoporous thin film. More particularly, the present disclosure is directed to highly corrosion and wear-resistant metal substrates, such as stainless steel, carbon steel, or aluminum, that have an oxidized surface and a metal oxide nanoporous thin film layer chemically bonded to the oxidized surface that is prepared from a nanoparticulate sol of titanium dioxide, silicon dioxide, zirconium dioxide, alumina or a combination thereof. The present disclosure is also directed at various methods for preparing the highly corrosion and wear-resistant metal substrates including the metal oxide nanoporous thin film.
In many applications that utilize metal substrates that are in contact with aggressive media such as corrosive liquids, the metal substrates can quickly become corroded and worn. One specific example of a metal substrate that can quickly become corroded and worn during normal usage is a metal plate heat exchanger. Metal plate heat exchangers are commonly constructed of stainless steel, although in applications where salt and chlorides are present, more expensive materials such as titanium or stainless steel alloys such as 254 SMO® may be utilized. More aggressive media such as sulfuric acid and the like may require special alloys. Generally, a stainless steel heat exchanger will have a life expectancy of less than about five years in many applications, and may be less than one year in particularly corrosive applications.
The amount of corrosion and wear on a metal substrate, such as a metal plate heat exchanger, shell and tube heat exchangers, or cooling towers, for example, is directly dependent upon three variables: (1) the metal comprising the metal substrate; (2) the surface roughness of the metal substrate prior to the application of any coating; and (3) the media that the metal substrate is exposed to. These variables need to be addressed and controlled in order to achieve the greatest life expectancy for the metal substrate during use.
As compared to other types of steel, stainless steel generally exhibits a lower corrosion rate in an aqueous environment due to the formation of a thin passivating oxide film that covers and protects the metal surface. Pitting corrosion occurs when passivity fails or is otherwise absent at localized points on the metal surface. Pitting corrosion is a particularly aggressive form of corrosion that is focused on a small area of the metal substrate. Pitting corrosion forms pits, which are holes formed on the metal surface. Pits tend to propagate very rapidly due to anodic dissolution of the metal. As such, pitting corrosion may be referred to as “localized.” A common and important type of pitting corrosion occurs on passivated iron-based alloys in contact with halide-containing solutions. Chloride is a common and aggressive halide anion and causes pitting corrosion in many metals and alloys.
Generation of corrosion pits on stainless steel immersed in aggressive media, such as a chloride solution, generally occurs in three distinct stages: nucleation, metastable growth, and stable growth. Many pits that nucleate do not propagate indefinitely. Instead, many pits re-passivate after a very short period of metastable growth. Metastable pits generally do not cause significant damage to the metal surface. The final diameter of metastable pits may be a few micrometers.
Pit growth is generally sustained by the development of a highly aggressive analyte, which involves a strong oxidizing solution having an oxidizing reduction potential of +600 to +1200 mV inside of the pit. The analyte also comprises an enhanced concentration of anions that migrate into the pit, which maintains analytic charge neutrality. Pit growth is self-sustaining due at least in part to development of the aggressive analyte.
Most pits generally tend to continue growing once they have become established. Therefore, the susceptibility of a metal to pitting corrosion is linked to the formation of stable pits. The resistance of stainless steel to pitting generally relates to the critical potential measurable by various electrochemical methods. The potentiodynamic method involves an applied potential scanned to noble values, whereby the respective current is measured.
The potentiodynamic method provides a measurement of the pitting potential (Ep) related to pit nucleation. The potentiodynamic curves depend upon experimental variables, such as potential scanning rate. Ep may be determined by extrapolating to the passive current density for the rising curve observed in the early stages of pitting. Ep values may also be determined from the same curve by relating to a predetermined current density and to a current density ten times higher.
Another accelerated way of testing for pitting corrosion resistance with respect to a surrounding medium is to expose the coated stainless steel material to an aggressive corrosion medium of interest. Periodic visual examination of the material permits qualitative ranking with respect to pitting corrosion susceptibility. Although such testing is more time consuming than the electrochemical methods, it may provide improved evaluation of longevity in a medium of specific interest.
Based on the foregoing, there is a need in the art for metal substrates that are constructed out of lower cost materials that can provide increased resistance against pitting and corrosion and thus a longer service life.